This invention is an improvement in the elastomeric bladder assemblies disclosed in co-pending, co-assigned patent application Ser. No. 312,430 filed Oct. 19, 1981 which is a continuation of Ser. No. 113,224 filed Jan. 18, 1980 for Medical Infuser by J. R. Peery, P. F. Carpenter and W. K. Griesinger and in U.S. Pat. No. 4,201,207 which in turn were improvements over the devices disclosed in U.S. Pat. Nos. 3,895,631 and 4,140,117 all patents being commonly owned. These devices all employed elastomeric collapsing bladders in medical infusers which were adapted to administer a pre-determined amount of pharmaceutically active material to a subject over a prolonged period of time. Since the flow rate of the pharmaceutical agent from these devices is dependent upon the pressure generated within the elastomeric bladder, it is important the pressure generated by the elastomeric bladder remain relatively constant throughout a substantial portion of the operating lifetime of the device. The above-described devices are generally satisfactory for their intended purposes and all represent significant advances over the art. Nevertheless, it has been observed that in the terminal portion of the dispensing cycle, a pressure increase or "spike" occurs just prior to the emptying of the contents of the bladder which causes an increase in the flow rate from the device immediately prior to the final tail off the fluid flow at the end of the duty cycle. While this increase in flow rate is acceptable in most applications, it could present a problem in those particular instances in which an extremely potent drug is being administered. Presently, the only way this increase in flow rate can be eliminated is to remove or change the bladder prior to complete emptying, with a concommitant loss of unused drug.
According to this invention, however, I have provided a means whereby the terminal pressure spike may be eliminated while still permitting total utilization of the drug within the bladder. Accordingly, it is an object of this invention to provide an elastomeric bladder assembly whch exerts a substantially constant pressure on its contents throughout its duty cycle. It is another object of this invention to provide means within a collapsing elastomeric bladder for maintaining the pressure generated within the bladder at a substantially constant rate throughout is entire collapse. These and other objects of the invention will be readily apparent from the following description of the invention with reference to the accompanying drawings wherein: